In the field of automobiles, construction machines, and the like, there has been intense research in how to reduce mass using a high-strength material. For example, in the automobile field, the amount of a high-strength steel sheet used has steadily increased for the purposes of suppressing an increase in the mass of a vehicle, which is required for securing collision safety and realizing high performance, and improving fuel efficiency to reduce carbon dioxide emissions.
In such an increase in the amount of a high-strength steel sheet used, the most significant problem is a phenomenon called “deterioration in shape fixability” which is inevitably caused when the strength of a steel sheet increases. “Deterioration in shape fixability” refers to the general term for a phenomenon in which the spring-back amount after forming increases along with an increase in strength; and thus, the desired shape is difficult to obtain. In order to solve a problem caused by such a phenomenon, a process (for example, restriking) which is unnecessary for a low-strength material (material having superior shape fixability or having no problem in shape fixability) may be added, or the shape of a product may be changed.
As a method of solving this problem, a hot forming method called hot stamping has attracted attention. In this hot forming method, a steel sheet (workpiece) is heated to a predetermined temperature (generally, a temperature at which the steel sheet is in the austenite phase) to reduce the strength (that is, to promote forming) and then is formed with a die at a lower temperature (for example, room temperature) than that of the workpiece. With such a forming method, a shape can be easily given to a workpiece and a rapid cooling treatment (quenching) can be performed using a difference in temperature between a workpiece and a die. Therefore, the strength of a formed product can be secured.
Regarding a steel sheet suitable for this hot stamping and a forming method thereof, several techniques are reported.
Patent Document 1 discloses a steel sheet from which a member having superior impact properties and delayed fracture resistance can be obtained after hot forming (corresponding to hot stamping) by controlling the amounts of contained elements and relationships between the amounts of elements to be in predetermined ranges.
Patent Document 2 discloses a method of obtaining a high-strength component by controlling the amounts of contained elements and relationships between the amounts of elements to be in predetermined ranges as described above; and heating a steel sheet in a nitriding atmosphere or a carburizing atmosphere before forming.
Patent Document 3 discloses means for obtaining a hot-pressed product at a high productivity by specifying chemical components and a microstructure and limiting heating conditions and forming conditions.
Recently, the utility of hot stamping has been widely recognized, and the application of hot stamping has been also discussed for various members. Such members include, for example, a long component such as a center pillar of a vehicle.
The present inventors found that a small but certain amount of deflection occurred in such a long component unlike a short component in which the deflection was negligible.
The present inventors presume the reason why the deflection occurs to be as follows: cooling conditions during hot stamping are deviated from ideal uniform conditions by an increase in the size of a component; and as a result, non-uniform strains are introduced into the component.
As a result of detailed investigation regarding the reason for such non-uniformity, the present inventors had the feeling that the non-uniformity of strains may relate to variation in the carbon concentration of a steel sheet immediately before hot stamping (immediately before forming using a die).
As a result of further study, it was found that, in a heating process immediately before forming, dissolution behaviors of iron carbides in a steel sheet are the key to suppressing the non-uniformity.
In general, a steel sheet for hot stamping includes the ferrite phase as a primary phase, pearlite and the like as a secondary phase, and a microstructure composed of iron carbides. In such a steel sheet, carbon, produced from iron carbides in the heating process before forming, is solid-soluted in the austenite phase, this austenite phase is quenched to be transformed into the martensite phase, and as a result, a high strength can be obtained. The strength of the produced martensite phase strongly depends on the cooling conditions and the concentration of carbon which is solid-soluted in the austenite phase. Therefore, it can be easily presumed that a method of uniformly dissolving iron carbides in the heating process has a strong effect on mechanical properties of a formed product obtained in the subsequent process.
Furthermore, as a result of detailed study, the present inventors found that the uniformity of the carbon concentration in the austenite phase was strongly affected not by the size (average size) of iron carbides before heating but by the size distribution thereof. However, there are no examples of investigating a steel sheet for hot stamping from this point of view.
Patent Documents 1 to 3 have no description regarding the size distribution of carbides.
Patent Documents 1 and 2 neither disclose under what conditions a cold-rolled steel sheet is annealed nor investigate the control of carbides in a steel sheet.
Patent Document 3 neither has description regarding heating history, which is most important for controlling a state of carbides during the annealing of a cold-rolled steel sheet, nor investigates the control of carbides.
Patent Document 4 discloses a technique in which a spheroidizing ratio and an average particle diameter of carbides are controlled to be in predetermined ranges, thereby obtaining a steel sheet having superior local ductility and hardenability. However, Patent Document 4 has no description regarding the size distribution of carbides.
In addition, in order to obtain a predetermined metallographic structure, extremely special annealing is necessary, and the production in a general type of continuous annealing equipment or continuous hot dip coating equipment is not considered. Therefore, in Patent Document 4, annealing conditions are not regulated to control the size distribution of carbides.
Patent Document 5 discloses a technique in which an average particle diameter of iron carbides is controlled to be in a predetermined range, thereby obtaining a steel sheet having superior strength stability to heat treatment conditions and superior delayed fracture resistance. However, Patent Document 5 has no description regarding the size distribution of carbides.
Patent Document 5 neither discloses heating history until the maximum heating temperature nor has description regarding the control of the size distribution of carbides.
Patent Document 6 discloses a technique in which an average particle diameter of ferrite and a ratio of spheroidal carbides having a predetermined size to all the spheroidal carbides are controlled to obtain a high carbon steel sheet having superior wear resistance. However, Patent Document 6 has no description regarding the size distribution of carbides.
In addition, in order to obtain a predetermined steel sheet, it is necessary that a hot-rolled steel sheet and a cold-rolled steel sheet be annealed for a long time multiple times. In addition, the production in a general type of continuous annealing equipment or continuous hot dip coating equipment is not considered. Therefore, in Patent Document 6, annealing conditions are not regulated to control the size distribution of carbides.
Patent Document 7 discloses a technique in which an average particle diameter of ferrite and an average particle diameter of carbides are controlled to obtain a medium or high carbon high-strength steel sheet having superior punching quality. However, Patent Document 7 has no description regarding the size distribution of carbides.
In addition, Patent Document 7 discloses a cold-rolled steel sheet in the as-cold-rolled state; and a cold-rolled steel sheet which is annealed under annealing conditions of a low temperature of 350° C. to 700° C. and a long time of 10 hours to 40 hours. In addition, the production in a general type of continuous annealing equipment or continuous hot dip coating equipment is not considered. Therefore, in Patent Document 7, annealing conditions are not regulated to control the size distribution of carbides.
As a result of study, the present inventors found that the size distribution of iron carbides had a close relationship with a change in the rise rate of a steel sheet temperature during the annealing of a cold-rolled steel sheet. However, there are no examples of investigating a method of producing a steel sheet for hot stamping from this point of view.